My Clueless First Friend (2018)

Blurb:
One lonely, gloomy fifth-grade girl is the target of her classmates’ relentless bullying and teasing—that is, until a new kid arrives on the scene. Friendly Takada is as clueless as he is well-meaning, but somehow he possesses the magic ability to start drawing “Grim Reaper” Nishimura out of her shell. As the elementary schoolers experience all the fun of a childhood summer together—from going to the pool to picking sunflowers to watching fireworks—an unusual friendship blossoms!

Source: www.anisearch.com/manga/44782